This invention relates to a circuit arrangement for high-frequency operation of a discharge lamp, comprising:
input terminals for connection to a low-frequency supply voltage source, PA1 low-frequency rectifying means for generating a DC voltage across first capacitive means from a low-frequency supply voltage delivered by the low-frequency supply voltage source, PA1 a DC/AC converter for generating a high-frequency AC voltage from the DC voltage, PA1 a load branch comprising a series arrangement of inductive means, second capacitive means, and coupling means for coupling the discharge lamp to the load branch, which load branch is coupled to the DC/AC converter, PA1 high-frequency rectifying means for converting a high-frequency voltage generated by the DC/AC converter into a DC voltage, which high-frequency rectifying means are coupled to the first capacitive means and to the load branch and comprise a series arrangement of first and second unidirectional means having the same orientation, and PA1 control means for controlling the power consumed by the discharge lamp to a level which is dependent on a control signal which is a measure for a desired power.
Such a circuit arrangement is known from WO 96/10897. The first rectifying means in the known circuit arrangement are constructed as a voltage doubler, and the first capacitive means across which the voltage doubler generates a DC voltage comprise a first and a second capacitive impedance. The voltage across the first capacitive means is also referred to as buffer voltage hereinafter. The load branch comprises besides the inductive means, the second capacitive means, and the coupling means, also further capacitive means. One side of the further capacitive means is connected to a junction point. A further side of the further capacitive means is connected to another junction point. The power consumed by the discharge lamp, also referred to as lamp hereinafter, can be controlled by control means which influence the duty cycle of the switching elements.
The first rectifying means are provided with first and second unidirectional means which also form part of the second rectifying means. The second rectifying means are to ensure that the circuit arrangement behaves substantially as a resistive impedance during lamp operation. The circuit arrangement will cause comparatively little radio interference in that case and will have a high power factor during lamp operation. This means that the buffer voltage must always be higher than a bottom value. When the voltage doubler is used, the bottom value is equal to the peak-to-peak voltage of the low-frequency voltage source. The bottom value is equal to the peak voltage if no voltage doubling takes place. The buffer voltage rises comparatively strongly in proportion as the adjusted power is lower in the known circuit arrangement. On the one hand this requires a dimensioning of the circuit arrangement such that the buffer voltage is higher than the bottom value during nominal operation. On the other hand, components such as the switching elements and the first capacitive means must be designed for high voltages, or the range over which the lamp power is controllable must be limited so as to avoid damage to said components.